Turkish police on Friday detained 18 academics who signed a petition criticising the military crackdown in the Kurdish-dominated southeast, thus angering President Erdogan and triggering new alarm over freedom of expression in the country.

In a rare rebuke to Washington's NATO ally, the US ambassador to Turkey expressed concern over the effect of the investigations against the academics on a healthy political debate over the violence in the southeast.

Western concern over freedom of expression in Turkey is already running high due to the detention since November 26 of two prominent journalists of the opposition daily Cumhuriyet.

Police had early Friday launched an operation to takeTurkish academics into custody as part of an investigation accusing them of disseminating "terror propaganda" by signing a petition denouncing military operations against Kurdish rebels.

In a dawn operation in the northwestern province of Kocaeli, police raided the houses of 19 academics, detaining 15 of them, Hurriyet Daily News reported.

In the northwestern province of Bursa, meanwhile, three academics were also detained, according to the Anadolu news agency.

Prosecutors on Thursday launched a vast investigation into over 1,200 academics from 90 Turkish universities for engaging in "terrorist propaganda" and "inciting hatred and enmity" by signing the petition.

Entitled "We won't be a party to this crime", the petition urged Ankara to halt "its deliberate massacres and deportation of Kurdish and other peoples in the region."

It accused the Turkish government of condemning towns in the Kurdish-dominated southeast “to hunger through its use of curfews that have been ongoing for weeks” and attacking them “with heavy weapons and equipment that would only be mobilised in wartime”.

“As a result, the right to life, liberty, and security, and in particular the prohibition of torture and ill-treatment protected by the constitution and international conventions have been violated,” it said.

Signatories included US philosopher Noam Chomsky as well as dozens of academics from around the world.

Erdogan has publicly criticised Chomsky but so far none of the foreign signatories have been put under investigation in Turkey.

More than 2,000 lawyers signed and published online a pledge to offer free legal assistance to the academics.

“The declaration does not praise or call for hate or committing crimes,” said one law professor, asking not to be named for fear he himself would be prosecuted.

All-out offensive

Turkey is waging an all-out offensive against the Kurdistan Workers' Party (PKK), with military operations backed by curfews aimed at flushing out rebels from several southeastern urban centres.

But Kurdish activists say dozens of civilians have died as a result of excessive force and the operations have become the subject of huge controversy in Turkish society.

Erdogan on Friday launched his strongest attack yet on the signatories, accusing them of supporting the Kurdish rebels and thus being a "party" to the crimes of the PKK.

"Those standing by the perpetrators of the massacres are a party to the crime," he told reporters after Friday prayers in Istanbul.

"Our people must understand who is who – having a PhD title doesn't necessarily make you an intellectual. These are people in the dark. They are cruel and despicable," the president said.

Critics have denounced the move as the latest attempt by Erdogan to stifle dissent, with pro-Kurdish People's Democratic Party (HDP) condemning "the steps taken by the Turkish government that are plunging Turkey into a deep darkness."

"These operations... which are only be seen in undemocratic regimes are very dangerous and unacceptable," the main opposition Republican People's Party (CHP) added in a statement.

US ambassador to Ankara John Bass expressed concern over the "pressure having a chilling effect on legitimate political discourse across Turkish society regarding the sources and solutions to the ongoing violence."

"In democratic societies it is imperative that citizens have the opportunity to express their view, even controversial or unpopular ones," he added in a statement.

University authorities on Friday also opened probes into 20 academics in Mersin University on the Mediterranean and four others in southeastern city of Gaziantep that could potentially lead to their dismissal.

Earlier on Thursday, Duzce University in northwest Turkey dismissed an associate sociology professor after she signed the declaration, Turkish media reports said.

The United States and EU have already expressed concern over the imprisonment pending trial of Cumhuriyet daily editor Can Dundar and the paper's Ankara bureau chief Erdem Gul for publishing articles alleging the government delivered weapons to Islamists in Syria.